The purpose of this study is to look at how some medications are processed (metabolized) in children with cystic fibrosis as compared to children without this disease. The medications we are using in this study, dextromethorphan and caffeine, will tell us how many other medications are processed in the body. If your child has cystic fibrosis, we will also look at how the genetic type of your child's cystic fibrosis (genotype) affects the way your child's body processes medications.